Sound-absorbing wall covering



Sept. 10, 1963 A. BoscHl ETAL ,103,

' SOUND-ABSORBING WALL COVERING Filed Dec. 12, 1961 United States Patent3,103,255 SOUND-ABSORBLNG WALL COVERING Antonio Boschi and GiancarloPennati, Milan, 'Italy, assignors to Societa Applicazioni GommaAnti'vibranti SAGA S.p.A., Milan, Italy Filed Dec. 12, 1961, Ser. No.158,776 Claims priority, application Italy Dec. 23, 1960 1 3 Claims.(Cl. 18133) This invention relates to the technique oi sound absorbing,tor example, in silencing workshops, ofiices, cinemas and other placeswherever noise is to be absorbed and acoustical reverberations are to beprevented.

Modern techniques in sound absorbing employ multipleresonator panelsbased on the principle of Helmholtzs resonators, which have a remarkablesound absorbing power. However, such panels are relatively expensive.Their great cost prevents their-use to any large extent in smallindustries and h-andicraftsman workshops, tor example.

A principal object of the present invention is to provide asound-absorbing covering or sound-absorber of the multiple-resonatortype, which is relatively cheap and at the same time possesses a highabsorbing power with regard to low and high acoustical frequencies.

Another object of the invention is to provide a soundabsorber suitableto be tuned to a relatively large band of low frequencies, according tothe circumstances, without prejudice for its absorbing power with regardto higher frequencies, superior to about 2000 c./s.

According tothe invention, the covering includes, in combination with aceiling or wall of a space or room, a rigid plate corrugated orundulated with substantially trapezoidal-wave shaped undulations. Theplate or undulated member is fixed to a Wall in parallel spaced relationto the latter thereby to provide parallel channels open towards the saidwall alternating with channels opening freely towards the space. Arelatively great number of apertures of relatively small areas in thebottom of each of the channels open towards the wall, and a relativelysmall number, of apertures of relatively large areas in the bottom ofthe channels open towards the space or. room with a filling of a fibrousmaterial in those. channels only which open towards the wall. I

The corrugated or undulated member is preferably moulded infibre-cement, but also other materials can be used, such as metals orsynthetic resins, provided they confer to the plate a sufficientindeformability under its own weight and that of the fillings.

According to an important and advantageous'teature of the invention,each outside flank or outermost portion of each extreme channel presentsa height' or cros-sasectional length greater than the depth of theremaining channels and ends with a fitting flange to the wall. The twoflanks provide therefore spacing members between the plate and the wall.

In case of plates of substantialdimensions, the bottom of anintermediate channel open towards the room can be heightened withrespect to the general level of the member to the level of the twofitting flanges thereby to provide for an intermediate anchorage of theplate to the wall. This is particularly advantageous in the case inwhich the plate is fixed to the ceiling of the room.

The spacing of the plate with respect to the wall. pro- 3,103,255Patented Sept. 10, 1963 2 vides a cavity between the plate and wallbridging a selected number of channels open towards the wall forming aresonant case which is common to all the apertures in the bottom of suchchannels.

The response to low frequencies depends on the number of theseapertures, their diameter and the volume of the cavity mentioned above.The volume of the spaces is in turn a function of the distance betweenthe plate and the wall. The greater the distance, the lower the meanfrequency of the absorbed band. Thus, by interposing between the fittingflanges on the plate and the wall suitable shims, not shown, it ispossible to tune the device according to the invention on any band oflow frequencies included between about 80 c./s. and about 2000 c./s.

Other features and advantages of the invention will 0 result from .thefollowing description with reference to the appended drawings, in which:

FIGURE 1 is a sectional perspective view of an embodiment of thisinvention applied to a ceiling of a room;

FIGURE 2 is a cross sectional View of another embodirnent.

1n the drawings, similar reference numbers indicate equal or equivalentparts.

Reference 10 indicates a ceiling of a room to which is secured by meansof nails 11 a plate 12 moulded in cement-asbestos. This material isfound particularly advantageous because, besides assuring to the plate agood self-support, it presents also a great inertia with regard toacoustical vibrations and so it seldom resonates. In other Words, thismaterial has a high inner friction, which damps every incipientacoustical vibration. In general, the plate thickness is between two andfour millimeters depending on its dimensions. The plate is rectangularin plan and is corrugated to a trapezoidal wave shape forming channels12a which open downwardly, that is towards the space or room in whichsound absorbing is to take place, alternating with channels. 12b whichopen towards the ceiling 10. In each channel 12b is placed a mineralfiber sliver 13 which fills its respective channel to a level not higherthan that of the bottoms 14 of the downwardly open channels 12a. Suchsliver is preferably glued to the walls of the respective channel bymeans of any suitable adhesive. The first channel on the right in FlGURE1 is shown without the respective fibrous filling, thereby to rendervisible two longitudinal rows of circular apertures 15 in the bottom 16of the channel. Such apertures are provided in all the channels 12b. Theapertures 15 have each a relatively small diameter and are relativelyclose to one another.

In the bottom 14 of each channel 12a open towards the room one row onlyof circular apertures 17 of a relatively large diameter is provided, theapertures 17 being appreciably spaced from one another. So, consideringthe volume of the channels 12b open towards the ceiling, each aperture15 has associated therewith a relatively small fr-action of such volume.On the contrary, considering the volume of the free space between theplate .12

and the ceiling 10, to each aperture 17 corresponds a relatively greatfraction, several times greater (in volume towards the room and each ofthem ends as a flange 20 by which the nails 11 fasten the plate to theceiling.

The height of the flanks 18, 19 determines therefore the smallestdistance between the plate and ceiling. The practical effects,considering the diameter and disrtibution density of the apertures 17 inthe bottoms 14 of the downwardly open channels 12a are such that thesmallest distance should be determined to obtain with regard torelatively low acoustical frequencies (40-2000 c./s.), the highestabsorption within the band adjacent the high limit of these frequencies,such as the band 8004000 c./s. for example. In this manner frequencieswhich are less than 800-c./s. will be absorbed somewhat lessefiiciently, which does not present, however, any inconvenience inpractice. So, for example, in a case in which the sound or noise to beabsorbed presents a maximum frequency of 200 c./'s., the distancebetween the plate 12 and the ceiling In such a way, the plates Inpractice, before application, the usual preliminary phonornetricmeasurements will reveal the band in the field of'low frequenciesprevailing in the room to be sound I insulated, whereupon the thicknessof the spacing listels may be determined. I

The plates lz can present relatively great dimensions, whenmeasuredtransversely of the channels. In such a case, to prevent theplates, from bowing downwardly, the flanks 22, 22 (FIGURE 2) of anintermediate channel 12a can extend upwardly so that the bottom of thischannel is at the level fo the terminal fastening flanges 20 of theplate, whereby said bottom too can be nailed to the ceiling Apparently,in comparison with multiple-resonator panels, the operation of thepresent sound-absorbing covering-is not based exclusively on theHelmholtzs resonance effect. Presumably a following interpretation isvalid.

Sound waves of a relatively high frequency are more directional thanthose of a relatively low frequency. Furthermore, absorption ofrelatively high frequencies does not involve apar-ticular selectivity ofthe absorbing medium or means: in other words, all the frequenciesbetween about 2000 c./s. and about 10,00015,000 c./s. are absorbedsubstantially to the same degree (in general satisfactory) by means of asuitable layer of mineral or vegetable fibers.

' In the specific case,'high frequency waves are received by the veryhigh number of apertures 15 and, subdivided into separate wave fronts,conveyed towards the heaps Q fibres 13; The directionality of highfrequency waves gives here rise to innumerable reflections anddeflections of the single wave fronts with consequent dissipation ontheir energyprincipally by way of the fibers and secondarily by way .ofresonators. formed by the channels 12b containing the fibers.

nant systems must be at least roughly tuned to that low frequency bandwhich prevails in the field 40-72000 c./s. I The channels, which aretrapezoidally shaped, allow extension remarkably of the limits of suchhand, without a notable drop in absorption factor, as is lrnown in theart. However, in. the. present case, the trapezoidal chanpress the lowfrequency waves, leading them towards therelatively large apertures17,'while the extinctionof such waves due to resonance is carried out bythe aircase formed between the plate 12 and the ceiling 10.

The air volumes delimitated by each bottom 14, the ceiling 10 and theimaginary planes indicated with 30 and 31 in FIGURE 1 define in thisair-case virtual primary resonators, in which a great proportion of theenergy contained in low frequency waves is extinguished. A part of thesewaves is reflected by the ceiling 10 towards the channels 12b opentowards the ceiling and which form together with the latter secondaryresonators, they too virtual because delirninated partly by theimaginary planes 31, 32 (FIGURE 1).

Of course, the exact position of the planes 30, 31, 32 can be differentfrom that shown for illustrative purposes. FIGURE 1 clearly shows thatthe volume of these virtual secondary resonators is greater than that ofprimary res onators, and this fact is very favourable to the effects ofthe present invention; in fact, due to the virtuality of the planes '30,31 deliminating the primary resonators, only the acoustical componentsstill having a certain directional character easily form stationarywaves between the bottom 14 and the ceiling 10, while the lessdirectional components escape and spread laterally. But such lessdirectional components have the lowest frequency among all frequenciespresent and the highests volumes of the virtual secondary resonators arejust the most suitable means for their absorption.

The above theory explains also both the necessity of a free spacebetween the plate 12 and the ceiling 10, which bridges a number ofchannels 12b, and the capacity of the present sound-absorbing coveringto absorb a large band of low frequencies even with an only roughtuning,

It is to be understood that the above theory does not limit the scope ofthis invention and should be considered only as an attempt of alogicalinterpretation of the phenomena discovered in laboratory tests onthe above described coverings.

What we claim is:

1. A sound-absorber comprising, an undulated member of substantiallyrigid sheet material having undulations defining a plurality of adjacentparallel channels of a substantially trapezoidalcross-sectional shape,all of said channels having a small base ridge defined by said undulatedmember and alternate ones of said channels having a large base areaopening in a common direction, said member having two outermost portionseach defining a r V for securing said sound-absorber to a wall orceilingde-' fining a space between theisound-absorber or wall, and thenels12a open towards the ambient. and destined tov manipulate the lowfrequencies. seem. to work as resonators in a very limited degreeonly;actually, they rather com- 'side of respective channel and each having alonger crosssectional length than the remaining, undulations of saidmembe each of said outermost portions having a. flange channels openingtoward said space comprising a soundabsorbing filling therein. 7

. 2. A sound-absorber comprising, an undulated member of substantiallyrigid sheet material having undulas tions defining a plurality ofadjacent parallel channels of a, substantially trapezoidal.cross-sectional shape, all of said channels having a small base ridgedefined by said undulated member and alternate ones of said channelshaving a large base area opening in a common direction, said memberhaving two outermost portionseach defining a side of a respectivechannel and each having a longer cross-sectional length than theremaining undulations of a said member, eachof said outermost portionshaving a flange fon securing said sound-absorber to. a wall: or;ceiling, defining. a. space between. the soundi-absorberor wall, andthe: channels opening toward said space comprising a sound-absorbingfilling therein, and the ridges of said undulations each having aplurality of spaced,

through apertures therein. 7

,3. A sound-absorber comprising, 'an undulated member of substantiallyrigid sheet material, having undula- 'tions defining a plurality ofadjacent parallel channelsof a substantially trapezoidal cross-sectionalshape, 'all' of said channels having a small baseyridge defined y Saidundulated member and alternate ones of said channe1s" 5 6 having a largebase area opening in a common direction, disposed substantially flushwith the ridges of the undulasaid member having two outermost portionseach detions defining the channels in which said filling is disposed.fining a side of a respective channel and each having 2 longercross-sectional length than the remaining undula References Cited in thefile of this Patellt tions of said member, each of said outermostportions hav- ,5 UNITED STATES PATENTS ing a flange for securing saidsound-absorber to a Wall or ceiling defining a space between thesound-absorber gif g 1g or wall, and the channels opening toward saidspace com- 2902854 Greene 1959 prising a sound-absorbing fillingtherein, the ridges of 'f' said undulations each having a plurality ofspaced through 10 FOREIGN PATENTS apentures therein, and saidsound-absorbing filling being 296,295 Switzerland Apr. 17, 1954

1. A SOUND-ABSORBER COMPRISING, AN UNDUALTED MEMBER OF SUBSTANTIALLYRIGID SHEET MATERIAL HAVING UNDULATIONS DEFINING A PLURALITY OF ADJACENTPARALLEL CHANNELS OF A SUBSTANTIALLY TRAPEZOIDAL CROSS-SECTIONAL SHAPE,ALL OF SAID CHANNELS HAVING A SMALL BASE RIDGE DEFINED BY SAID UNDULATEDMEMBER AND ALTERNATE ONES OF SAID CHANNELS HAVING A LARGE BASE AREAOPENING IN A COMMON DIRECTION, SAID